Another year of near-record global temperatures and the recent heat spike in major Australian cities should serve as warning of the vulnerability of populations to global warming, a leading climate researcher said.

The first complete datasets of land and sea surface temperatures worldwide, compiled by Europe's Copernicus Climate Change Service, indicate 2017 notched a fourth consecutive year of "exceptionally warm" conditions.

2017 was warmer than 2015 and 2014 – two years that had previously set the high mark for global heat in successive years, the preliminary set showed.

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Even without an El Nino to bump up temperatures, 2017 was one of the hottest couple of years globally on record.Credit:Katherine Griffiths

The average air temperature at the surface exceeded 14.7 degrees, the Copernicus service said. The most above-average temperatures for the year were in the Arctic, with few land regions below average. (See chart below.)

A slew of agencies will release their findings on January 18, but the ranking for 2017 is "very likely to be in the top three", Blair Trewin, senior climatologist at the Bureau of Meteorology, said.

Hottest non-El Nino year

Andy Pitman, director the new ARC Centre of Excellence for Climate Extremes based at the University of NSW, said global temperatures shouldn't be close to records since last year did not have an El Nino to give them a boost.

"Normally El Ninos add three-tenths of a degree or so to the global mean, and we're not in one," Professor Pitman said.

"Climatologically, it's really quite surprising" 2017 was so warm, and the record for a non-El Nino year, he said.

The implications, though, are unclear.

Penrith residents in western Sydney on Sunday were in the hottest place on the planet.Credit:Brook Mitchell

One more favourable view is temperature increases linked to rising greenhouse gas emissions may be catching up to trend after a relatively slow stint of increases at the start of the century, he said.

A less promising outcome, however, might be the climate is more sensitive to warming than the Intergovernmental Panel on Climate Change has estimated. The current range is about 2-4.5 degrees warming for each doubling of carbon dioxide in the atmosphere.

"It's optimistic to think that the climate sensitivity is at the lower end of the range," Professor Pitman said. "It's likely to be on the mid- to high-end of the range – which isn't good."

From averages to extremes

The weekend burst of heat across south-eastern Australia – including 47.3 degrees on Sunday in Penrith, the second hottest ever recorded in the Sydney Basin – underscores the risk of worse extremes when average temperatures rise.

While 2 degrees average global warming may not sound like a huge change, it implies land temperatures will rise about 3 degrees.

Those land regions in the mid-latitudes – where many Australian cities lie – may rise 4-5 degrees or more before extremes are taken into account, Professor Pitman said.

"If western Sydney warms by 4-5 degrees on the average in the summer, you're looking at extremes being 5-10 degrees higher than they currently are," he said.

The toll on the health of the population would mount as, if models predict, the length, frequency and intensity of heatwaves increase.

"What a few degrees increase in the average means is a dramatic escalation in the risk of extremely hot temperatures, lasting for longer periods of time," Professor Pitman said. "That places immense stress on emergency services, health services, the electrical grid, et cetera."

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While western Sydney had posted one hotter day – 47.8 degrees back in January 1939 – the population was not highly reliant on air-conditioners with houses much better adapted to passive cooling back then.

"We are now adapting to the pleasure of air-conditioning – and it's totally dependent on the power grid surviving," he said.